Massive Baker Cyst Resulting in Tibial Nerve Compression Neuropathy Secondary to Polyethylene Wear Disease
Symptomatic synovial cyst formation is an infrequent, late complication after total knee arthroplasty. Most often, these cysts are found incidentally. However, rarely they may become larger leading to significant pain and disability. The formation of gigantic cysts necessitating revision knee surgery has been detailed in a few case reports. To the author’s knowledge, this is the first report in the medical literature that describes peripheral neuropathy of the tibial nerve secondary to a massive Baker cyst after total knee replacement.
Postoperatively, all intraoperative cultures were negative, and the patient was followed closely at 1 week, 2 weeks, 6 weeks, and 3 months after the surgery. At her latest follow-up, the posterior leg compartment remained decompressed and her progressive sensory deficit had nearly resolved. Moreover, the left leg and posterior knee pain completely resolved.
Discussion
A leading cause of TKA failure is attributed to aseptic loosening from polyethylene wear disease.2 Implanted high-molecular-weight polyethylene (HMWPE) liners are known to undergo a variety of mechanical wear patterns within the knee. Observed patterns include pitting, scratching, burnishing, scratching, and delamination, which can all liberate numerous fine polyethylene particles.3 This wear debris induces macrophage phagocytosis that triggers an inflammatory reaction within the knee joint and can lead to synovitis, repeat effusions and, ultimately, to aseptic loosening.
Prior to 1996, polyethylene used in total knee replacement underwent a sterilization process in air. This oxygen-rich environment led to the development of free radical formation within the HMWPE. Ultimately, this had a detrimental effect on the polyethylene, leading to the formation of increased wear debris.4
Subsequently, orthopedic companies have changed their sterilization and manufacturing methods. Polyethylene components now undergo a variety of processes to eliminate or reduce oxidation, free-radical formation, and mechanical wear debris. Now, sterilization typically takes place in an inert atmospheric environment. Modern HMWPE implants often undergo higher irradiation to induce mechanical cross-linking, followed by either a re-annealing or remelting step. In other cases, manufacturers “dope” their polyethylene with vitamin E to quench free radicals within the material. While these steps have reduced the number of in vitro wear particles, the problem of wear debris, subsequent osteolysis, and aseptic loosening has not been eliminated.1-5
Polyethylene wear debris within the synovial fluid or tissue of failed TKAs can be identified with scanning electron microscopy or by light microscopy utilizing polarized light.1 In this particular case, wear debris was confirmed within the synovial tissue and in the fluid of the Baker cyst by microscopic analysis.
Formation of a popliteal or Baker cyst as a result of polyethylene wear disease is an infrequent but known complication of TKA. Reports have demonstrated variable success in cyst eradication when revision surgery is performed on knees with synovial cysts. Most of these reports indicate that cyst formation tends to occur as a late complication (7 or more years) after TKA.6-12
Treatment options may include skillful observation with close follow-up or revision surgery. Polyethylene exchange with synovectomy when feasible, as well as component revision with or without excision of the synovial cyst, are surgical options.
Niki and colleagues13 described a gigantic popliteal synovial cyst caused by wear particles after TKA. In this report, the surgeon performed a synovectomy and polyethylene liner exchange with retention of prosthetic components. At 12-month follow-up, the patient was reported to be doing well.
Mavrogenis and coauthors14 reported a wear debris–induced pseudotumor in the popliteal fossa and calf after TKA. In this case, in addition to the synovectomy, the surgeon removed all prosthetic components and used a semi-constrained implant to revise the knee. At 30-month follow-up, the patient reported having a painless knee.
While case reports have indicated that revision TKA for large, painful synovial cysts is a reasonable treatment option in carefully selected patients, there is a paucity of literature on this subject. Moreover, the present case appears to be the first literature report of a tibial nerve compressive neuropathy secondary to a synovial cyst after TKA.
Conclusion
In this report, polyethylene wear disease after TKA resulted in a massive synovial cyst extending into the posterior compartment of the leg. A progressive peripheral neuropathy confirmed by electromyography was also discovered. The patient underwent revision of a loose patellar component and worn polyethylene liner with complete synovectomy plus decompression of the cyst via needle aspiration. This resulted in an excellent short-term outcome with resolution of pain and significant improvement of the peripheral neuropathy 3 months after surgery.
